1. Field of the Invention
The invention relates in general to a system for identifying an optical signal and more particularly, to a system for determining the wavelength of an optical signal.
2. Description of the Related Art
The emergence of the laser as a potent source of controllable optical energy has made the rapid and accurate determination of the wavelength of the optical source a necessity in many military and commercial applications. For instance, in certain electronic warfare applications, the jamming of enemy optical return signals beamed at a particular target requires the accurate and quick identification of the enemy's incoming signal so that a jamming signal having the same wavelength can be scattered back to confuse the enemy's weapon guidance systems. In some communication applications, the quick and accurate identification of particular optical wavelengths is important in high density optical wideband networks which need to closely control the wavelength or frequency of the transmitted signals. A rapid determination of wavelength is also needed in a communications network in which a number of sets of remote sources of radiation are synchronized to a central set of reference optical sources. Other applications that require quick and accurate identification of the wavelength of an optical signal include the spectral line identification of laser outputs and the monitoring of stimulated emissions from a process or material for quality control purposes.
Traditionally, the determination of the wavelength component of an incident optical signal is performed using narrow band filters. However, the resolution of prior spectral discrimination systems depends on the width of the filter bandpass regions. For instance, an increase in resolution of spectral discrimination requires that the bandpass regions for each filter be narrow, resulting in an increase in the number of filters in order to analyze the same wavelength spectrum. Thus, the traditional technique requires a large number of narrow band filters to cover every possible wavelength of the incoming signal transmitter.
U.S. Pat. No. 5,225,894, issued on Jul. 6, 1993 to Nicholson, et al., hereby incorporated by reference, proposes a system for determining wavelengths using a limited number of broadband filters with overlapping bandpass regions to cover the wavelength spectrum of interest, but not with the resolution of a large number of narrowband filters. As asserted in the Nicholson patent, an array of apertures capture incident optical pulses and transport them either to broadband filters or directly to a photodetector. The output from the filters are then connected to optical delay lines before being directed to a photodetector, while the outputs from certain fibers (non-delay fibers) are connected directly to another photodetector. The time difference between when a signal arrives over optical delay lines from the filters and when a signal arrives over optical lines directly from the apertures are then reconverted mathematically into wavelength information.
It is desirable to eliminate the use of broadband filters to minimize the size, weight, and cost of the spectrometer. Further, the passing of an optical signal through the filters before going to the photodetector may cause the signal to be attenuated differently than the signal transported directly to the photodetector, as the filters are not made from the same material as the fibers that conduct the optical signal. Thus, certain compensation has to be made in the control electronics to handle the differential dynamic range of the delayed signal and the non-delayed signal. Thus, it is desirable that discrete filters be eliminated.